hysteresis
Summary
TLDRThis video explains the phenomenon of hysteresis in soils, where the relationship between soil moisture content and suction varies depending on whether the soil is draining or wetting. It uses an experiment with soil and water tanks to demonstrate how different pore sizes control the drainage and refilling processes. Larger pores drain first as suction increases, while smaller pores refill first when suction decreases. The difference in behavior during these processes results in distinct soil moisture retention curves, which reflect the history of wetting and drying cycles.
Takeaways
- 🌊 Hysteresis occurs during the wetting and draining of soils, where the relationship between moisture content and suction (negative pressure) changes depending on the soil's history.
- 🧲 The term 'hysteresis' refers to this dependence on the drying and wetting history of the soil.
- 🌧️ Hysteresis significantly impacts runoff generation due to differences in soil moisture content during drainage and refilling.
- 🧪 The experiment demonstrates hysteresis by controlling suction in soil using the height difference between two tanks, which simulates soil draining and refilling.
- 🔬 Suction breaks down surface tension in soil pores during drainage, with larger pores draining first and smaller pores draining as suction increases.
- 🔄 Narrow pore constrictions control the process of drainage as water retreats until it encounters another constriction, illustrating a microscopic view of the soil.
- 🌿 During refilling, capillary forces draw water back into the soil, with larger pore diameters determining when water fills the pore space.
- 🚰 Refilling occurs step by step, with larger pores filling first, and smaller pores only filling when suction reaches a certain threshold.
- 🌀 The hysteresis phenomenon is caused by the different mechanisms controlling drainage (small pores) and refilling (large pores), leading to variations in moisture content.
- 📈 The resulting graphs, known as soil moisture characteristic curves, show the distinct drainage and wetting patterns, highlighting the hysteresis effect.
Q & A
What is hysteresis in the context of soil moisture content?
-Hysteresis refers to the phenomenon where the relationship between soil moisture content and negative pressure head (suction) differs depending on whether the soil is undergoing drainage or wetting. The moisture content at a given suction depends on the history of drying or wetting.
What causes the difference in the moisture content between draining and wetting?
-The difference in moisture content is caused by the fact that during drainage, narrow constrictions in soil pores control water movement, while during wetting, larger pore openings control the entry of water. This results in different moisture content at the same suction level, depending on whether the soil is draining or wetting.
How does suction influence soil moisture content during drainage?
-During drainage, as suction increases (due to the water tank being lowered), larger pores in the soil drain first because surface tension forces in these pores break down more easily. As suction increases further, smaller pores also drain, leading to a decrease in overall soil moisture content.
What role do capillary forces play in soil moisture retention?
-Capillary forces, resulting from the surface tension between water and soil particles, help retain water in soil pores. During drainage, these forces need to be overcome by increasing suction, which breaks the surface tension and causes the water to drain out of the pores.
How does the refilling process (wetting) differ from the drainage process in soils?
-During refilling (wetting), larger pore openings control water movement, with water being drawn into the largest available pores by capillary forces. This is the opposite of drainage, where smaller constrictions dominate water retention and drainage.
What determines when a pore fills with water during the wetting cycle?
-During the wetting cycle, the largest pore diameter in the soil structure determines when water enters. Once the suction decreases to a level corresponding to that large pore size, water fills the pore and is pulled upwards until it encounters the next larger diameter opening.
Why do some pores drain immediately after surface tension is broken during drainage?
-Once the surface tension at a narrow constriction is broken, water retreats from the pore until it encounters another narrower constriction. This is because the pore is no longer able to sustain water through capillary forces, leading to immediate drainage.
What is a soil moisture characteristic curve, and what does it illustrate?
-A soil moisture characteristic curve, also known as a soil moisture retention curve, illustrates the relationship between soil moisture content and suction during the processes of drainage and wetting. It highlights the differences in moisture content depending on whether the soil is drying or refilling.
How does pore size affect water movement during drainage and refilling?
-Pore size influences water movement differently during drainage and refilling. During drainage, smaller constrictions control when water exits the pore. During refilling, larger pore openings control when water enters and how it moves upwards in the soil structure.
What is the significance of hysteresis in environmental processes like runoff generation?
-Hysteresis can significantly affect environmental processes such as runoff generation. The differing moisture content between drainage and wetting cycles influences how water is retained or released by soils, which in turn impacts the rate and volume of surface runoff.
Outlines
🌍 Understanding Hysteresis in Soil Moisture
This paragraph introduces hysteresis, a phenomenon occurring during the drying and wetting of soils. It explains how the characteristic curve of moisture content and suction pressure differs between drainage and wetting phases. The term 'hysteresis' originates from the dependence on the soil's wetting and drying history, which impacts processes like runoff generation.
💧 Soil Moisture Experiment Setup
This part describes an experiment demonstrating hysteresis in soil. It involves two tanks: one containing soil and the other controlling the suction pressure by adjusting its water level. The goal is to show the relationship between soil water content and negative pressure during soil draining and refilling. The tank's manipulation causes changes in suction, affecting the soil's water content.
🔬 Microscopic View of Soil Pores
Here, the script zooms in to examine the soil's pore structure. The graph on the left shows the average relationship between pressure head and moisture content for all pores in the container. The pores, seen under a microscope, respond to changes in suction, showing how capillary forces and surface tension affect drainage and moisture content.
🌡 Capillary Forces and Soil Drainage
This section delves deeper into how capillary forces and surface tension impact larger and smaller pores during the drainage process. As suction increases, larger pores drain first, followed by smaller ones. The draining process is controlled by narrow pore constrictions, which break down the surface tension and cause water to retreat, as shown in the animation.
🌀 Refilling the Soil and Water Entry
The refilling phase is explained in this paragraph, focusing on how water re-enters the pores. The largest pore diameters control when water enters, and capillary forces pull the water upwards through the pore spaces. The water moves one pore at a time, stepping up to larger openings until it fills the entire pore.
🔄 Variations in Wetting and Draining Patterns
This section highlights differences in pore structures and how they influence wetting and drainage patterns. Larger pores refill first, while smaller constrictions control drainage. The paragraph emphasizes how these variations in pore size create differences in moisture content based on the soil's drying or wetting history.
📉 Soil Moisture Retention Curves
Finally, the script explains how graphs called 'soil moisture characteristic curves' or 'soil moisture retention curves' illustrate the differences in moisture content between wetting and drainage phases. These graphs visually represent the impact of hysteresis on soil moisture at varying suction levels.
Mindmap
Keywords
💡Hysteresis
💡Soil Moisture
💡Suction
💡Capillary Forces
💡Pores
💡Soil Matrix
💡Drainage Curve
💡Wetting Cycle
💡Surface Tension
💡Soil Moisture Retention Curve
Highlights
Hysteresis is a phenomenon occurring during the draining and wetting of soils, where the relationship between moisture content and suction varies.
Hysteresis depends on the history of drying and wetting, significantly influencing runoff generation.
An experiment is conducted with soil in a tank to illustrate hysteresis, controlling soil suction by adjusting the height of a water tank.
The difference in height between the soil container and water tank dictates the suction in the soil.
A graph is used to represent the relationship between soil water content and negative pressure head during suction changes.
The soil is initially fully saturated, and suction increases as the water tank is lowered.
Larger pores drain first as suction increases due to the breakdown of surface tension forces, leading to decreased soil moisture content.
Further suction increase drains smaller pores, reducing soil moisture content even more.
Drainage of pores is controlled by the narrow constrictions in each pore, where surface tension must be broken for water to retreat.
During refilling, capillary surface tension forces draw water back into the pore spaces, but large pore diameters control this process.
Water enters the largest pore first and then moves upwards until it encounters the next larger opening.
Different pore structures affect water movement during the wetting cycle, as seen in the soil matrix's pore pathways.
The difference in controlling factors during draining (small constrictions) and wetting (large openings) results in varying moisture content at the same suction.
The moisture content depends on the history of draining or wetting, highlighting the nature of hysteresis.
Graphs illustrating the varying wetting and drainage patterns are known as soil moisture characteristic curves or soil moisture retention curves.
Transcripts
hysteresis is a phenomenon that occurs
during the draining and wetting of soils
the characteristic curve describing the
functional relationship between moisture
content and negative pressure head
referred to as suction is different
depending on whether the soil is
undergoing drainage or wedding
the name hysteresis is derived from
dependence on the history of drying and
wetting hysteresis can have a
significant influence on the generation
of runoff in this animation we
illustrate one mechanism for hysteresis
in a soil draining and refilling
experiment the tank on the Left contains
soil the suction in this soil is
controlled by lowering and then raising
the tank on the right the height
difference between the soil and the
water tank controls the suction in the
soil on the Left we will graph the
relationship between soil water content
and negative pressure head for the
experiment where suction is increased
and then decreased let's zoom in to view
the pore structure of the soil in the
soil container our graph on the Left
will show the relationship between
pressure head and moisture content as an
average of all the pores in the soil
container a macroscopic perspective our
view of the pore structure is a
microscopic perspective in the beginning
of the experiment the soil is completely
saturated as the water tank is lowered
the difference between the water level
in the two containers increases and the
suction in the soil increases there is a
relationship between pore size and the
suction that a pore can sustain through
capillary forces due to the surface
tension between the water and the soil
the increasing suction breaks down the
surface tension forces in the larger
pores they drain resulting in a decrease
in the soil moisture content as suction
increases the smaller pores drain and
moisture content decreases further the
drainage of pores is controlled by the
narrow
constrictions and the pores when the
surface tension at each narrow
constriction is broken water immediately
retreats from the pore until a narrower
constriction is reached as illustrated
in the animation remember we are
representing only four of millions of
pores in our soil sample the aggregate
effect over all the pores is the
draining curve during refilling when the
suction is being reduced due to the
water tank being raised capillaries
surface tension forces draw water back
into the pore space however now it is
the largest pore diameter that controls
when water enters a pore and immediately
is pulled upwards until the next larger
diameter opening is encountered as you
watch the wedding cycle notice an hour
cartoon of the soil matrix pores the
first two pathways have larger pore size
at the top and smaller at the bottom the
second two pathways have smaller pores
at the top and larger pores at the
bottom the water steps up one pore at a
time in the pore space on the left
however in the pore space on the right
entry of water into the lower pour does
not occur until the suction
corresponding to the large pore diameter
is reached then these pores fill to the
top immediately because there are no
larger pore openings above this
difference between the small
constrictions controlling drainage and
large openings controlling refilling
results in differences in moisture
content between draining and wetting at
the same suction the moisture content
depends upon the draining or wetting
history hence the name hysteresis the
graphs which illustrate the different
wetting and drainage patterns are called
soil moisture characteristic curves or
soil moisture retention curves
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